The present invention relates to a power transmission device that includes a torque feedback mechanism to change the position of a shift gear so as to change the speed of the transmission device. The speed of the transmission device is automatically switched to a proper value when load changes.
A conventional power transmission device, especially for electric spinning tools, such as electric drills and electric screwdrivers, includes a multiple-stage power transmission. A speed reduction mechanism is incorporated to provide multiple speeds associated torque change in accordance with the multiplicity of stages. Generally, the speed reduction mechanism is composed of a planetary gear system and clutch or driving members that are manually controlled to switch the speed between the multiplicity of stages. Due to the manual control, an operator has to judge the situation of the tool and decide when to activate the speed reduction mechanism in order to obtain desired torque or speed. However, manual operation is apparently not a feasible way to optimize the operation efficiency of the driving motor.
Therefore, it is desired to have an automatic mechanism for switching the speed of a transmission device based on load torque in order to optimize the operation of the transmission device.
In accordance with an aspect of the present invention, there is provided an automatic speed switching mechanism for a power transmission device, which comprises a torque feedback mechanism. The torque feedback mechanism includes a pushing wheel and a sliding ring engaging the pushing wheel. A C-shaped clamp and a compression spring which is fit over the frame and retained between ridges of the frame and the clamp. The angular position of the pushing wheel is limited by a torsion spring that is fixed to the frame. The sliding ring is limited to be moved axially in the frame. The transmission mechanism has a shifting gear which has inner teeth engageable with first planet gears and second planet gears. The shifting gear has an annular groove with which a plurality of pins on the clamp engage so as to retain the shifting gear in a first, high-speed low-torque stage while the shifting gear is engaged with the two planet gears, or retain the shifting gear in a second, low-speed high-torque stage and only engaged with the second planet gears. When the load torque on the pushing wheel is smaller than the force of the torsion spring and compression spring, the shifting gear is retained at the first stage and co-rotates with the two planet gears. When the load torque is larger than the force of the torsion spring and compression spring, it rotates and pushes the sliding ring by the inclined faces so that the sliding ring pushes the shifting gear which is in the second stage and cannot rotate due to the engagement of the protrusions of the frame and the notches of the shifting gear. The speed reduction mechanism of the transmission mechanism automatically shifts the speed reduction mechanism when the load torque increases or reduces so that the mechanical efficiency of the transmission device can be increased.
The present invention will become more obvious from the following description when taken in connection with the accompanying drawings, which show, for purposes of illustration only, a preferred embodiment in accordance with the present invention.